1. Field of the Invention
The present invention relates to an operational amplifier, more particularly to an operational amplifier able to be used under conditions where there are a plurality of types of power sources in existence, such as in a telephone exchange.
For example, the subscriber line interface circuits or trunk circuits of time division exchanges have been increasingly formed by electronic circuits in recent years, with frequent use being made of operational amplifiers and replacement of conventional coils, transformers, and other electromagnetic components with semiconductor circuits. In such case, it is necessary to supply a speech current to the subscriber's telephone set just as in the past, so that conventional interface conditions have to be maintained as they are. That is, the electronic circuits are placed under what is for them considerably a high voltage environment of a power source voltage of -48 V. The present invention concerns an operational amplifier suitable for operation under voltage environments different from general use.
2. Description of the Related Art
The operational amplifier spoken of in the present invention is, for example, driven by a power source V.sub.BB having a negative voltage with respect to the ground GND. This power source V.sub.BB is, in a exchange as mentioned earlier, -48 V. Since general purpose operational amplifiers are driven by voltages of about 36 V, this type of operational amplifier is not readily available commercially and is higher in cost. Further, it has several problems technically. Details of these problems will be discussed later, but briefly the first of these problems is that the voltage range of the input signal of the operational amplifier is narrow, the second is that the output signal of the operational amplifier only reaches a level about 1.0 V lower than the level of the ground GND, the third is that the Early effect causes the operational amplifier to become unstable in operation, and the fourth is that the idling current of the operational amplifier is large.